1. Field of the Invention
The present invention relates to proximity sensors.
2. Background Art
Switches are typically electrically controlled two-state devices that open and close contacts to effect operation of devices in an electrical or optical circuit. Relays, for example, typically function as switches that activate or de-activate portions of electrical, optical or other devices. Relays are commonly used in many applications including telecommunications, radio frequency (RF) communications, portable electronics, consumer and industrial electronics, aerospace, and other systems. More recently, optical switches have been used to switch optical signals (such as those in optical communication systems) from one path to another.
Although the earliest relays were mechanical or solid-state devices, recent developments in micro-electro-mechanical systems (MEMS) technologies and microelectronics manufacturing have made micro-electrostatic and micro-magnetic switches possible. MEMS switches enjoy the low signal loss and good isolation associated with mechanical switches, and the high switching speeds, low power consumption, and compactness of semiconductor switches.
Micro-magnetic switches typically include an electromagnet that, when energized, causes a cantilever to make or break an electrical contact. Because the switching function depends upon movement of a cantilever, MEMS switches must be packaged so that the cantilever is free to move to perform its function. Often this precludes the use of conventional microelectronic packaging techniques for MEMS devices, or requires that these techniques be modified. Such packaging considerations can complicate fabrication processes and increase costs.
Conventional micro-magnetic switches have other disadvantages. Typically, a spring or other mechanical force is used to restore the cantilever to its quiescent position when the electromagnet is deenergized. Thus, such switches are characterized as having only a single stable position (i.e., the quiescent state) and lack a latching capability (i.e., they do not retain a given position when power has been removed from the electromagnet). Furthermore, the spring required to restore the cantilever to its quiescent position can degrade or break over time.
Non-latching micro-magnetic switches are known. These switches include a permanent magnet and an electromagnet. The electromagnet is used to generate a magnetic field that intermittently opposes the field produced by the permanent magnet. Thus, the electromagnet must consume power to maintain the cantilever in at least one of the available positions. The power required to generate this opposing field can be significant. Such power requirements can reduce the desirability of such switches for use in space, portable electronics, and other applications that demand low power consumption.
The basic elements of a latching micro-magnetic switch include a permanent magnet, a substrate, an electromagnet, and a cantilever. The cantilever is at least partially made of a soft magnetic material so that the cantilever can retain a given position when power has been removed from the electromagnet. In an optimal configuration, the permanent magnet produces a static magnetic field that is substantially perpendicular to the plane of the cantilever.
Generally, proximity sensors are devices that include circuitry for sensing change in a magnetic, electric, or optical field. In most applications, these proximity sensors are designed to only detect when an object is in a general area of the sensor, but have no other functionality. Typically, these sensors are not sufficiently versatile to provide a proximity sensing for a variety of different applications.
However, the magnetic field lines produced by a regularly shaped permanent magnet (e.g., disk, square, etc.) may not necessarily be perpendicular to the plane of the cantilever. This is especially likely near the edges of the permanent magnet. Components of the magnetic field produced by the permanent magnet that are not substantially perpendicular to the plane of the cantilever can eliminate one of its bistable positions or greatly increase the current that is needed to switch the cantilever from one position to another.
Therefore, what is needed is a micro magnetic proximity sensor that is versatile and can be used in a variety of applications with only slight modification, that is relatively easy to fabricate and use, that can sense very small or very short distances, and that is capable of sensing direction of movement, distance, proximity, velocity, acceleration, and other relative characteristics between an detected object and the sensor.